In a vehicle transmission, hydraulic cylinders are used as shift actuators and as select actuators for driving a gear shift mechanism (e.g. Japanese Patent Application Hei 5-17243 published by the Japanese Patent Office in 1994).
In this disclosure, hydraulic cylinders for shift and select operation are controlled by fluid pressure supplied via a solenoid valve by a microcomputer, and when the vehicle issues a speed change request, the hydraulic cylinders drive a gear shift mechanism to a required position.
In this case, a three stage positioning function is required of the hydraulic cylinders. A conventional cylinder which permits three stage positioning is shown in FIG. 13.
Two free pistons 211, 212 are housed in the cylinder 206, and a piston 210 is accommodated between them. The piston 210 is fixed to a rod 201 passing through the cylinder 206. A pressure chamber 202 facing the free piston 211 and a pressure chamber 203 facing the free piston 211 are provided inside the cylinder 206, these pressure chambers 202, 203, being connected to a high pressure air supply via solenoid valves 204, 205.
In the state shown in the figure, when high pressure air is supplied to the pressure chamber 202 via the solenoid valve 204 and the pressure chamber 203 is opened to the atmosphere via the solenoid valve 205, the free piston 211 and piston 210 are displaced due to the pressure acting on its pressure-receiving surface.
The free piston 211 stops in an intermediate position shown in the figure corresponding to a midway stage, but the piston 210 displaces to the right of the figure until it comes in contact with the right-hand end of the cylinder 206.
When high pressure air is supplied to the pressure chamber 203 via the solenoid valve 205 from this state, and the pressure chamber 202 is opened to the atmosphere via the solenoid valve 204, the free piston 212 and piston 210 displace together to the left due to the pressure acting on the pressure-receiving surfaces of the free piston 212 and piston 210.
When the intermediate position (neutral position) shown in the figure is reached, the free piston 212 comes in contact with a step and stops in that position, but the piston 210 continues moving to the left together with the other free piston 211 until it comes in contact with the left-hand end of the cylinder 206.
On the other hand, when high pressure air is simultaneously supplied to the pressure chamber 202 and pressure chamber 203 via the solenoid valve 204 and solenoid valve 205, the piston 210 displaces to the neutral position together with the free pistons 211, 212, and stops in this position.
In this case, the rod 201 can be positioned in three stages by opening and closing the solenoid valves 204, 205, i.e. a maximum extension amount and minimum extension amount, and an intermediate position (neutral position) between these extremes.
However when the pressure chambers 202, 203 are opened to the atmnosphere after stopping the solenoid valves 204, 205 in the neutral position, and there is a difference in the response of the solenoid valves 204, 205 or pressure losses in the passages, or there is a difference in the capacities of the pressure chambers 202, 203, a pressure difference is easily established on either side of the piston 210 so that the working pressure acting on the piston 210 is unbalanced, and the piston 210 therefore moves or "drifts" towards the left or right.
To correct this drift, a high-speed response solenoid may be used, a throttle to adjust unbalance of pressure drop may be provided in a passage, or the resistance of a load connected to an output shaft may be added.
There is some scatter in the response speed of solenoid valves, and their response speed may vary according to the supply voltage and supply pressure. An attempt is often made to resolve this problem by providing a high pressure air discharge passage, increasing the sliding resistance of the piston, or using a control system which allows for drift.
However, some fluctuation of the vehicle battery voltage cannot be avoided. Decreasing the resistance of passages or reducing the left-right difference between pressure chambers requires the design layout to be symmetrical. This limits the degree of freedom of design, and necessarily makes the hydraulic cylinder larger and heavier.
In a control system wherein such drift is assumed to occur, during a select operation in the neutral position, it is first necessary to hold the neutral position by a shift hydraulic cylinder. This introduces a delay into the control, and also increases the consumption amount of compressed air.
Also, when a transmission had one reverse gear and seven forward gears, a hydraulic cylinder was required which could be positioned in three stages as a shift actuator, and a hydraulic cylinder was required which could be positioned in four stages as a select actuator.
If two types of hydraulic cylinder are provided, as they respectively have different components, manufacturing costs are increased compared to the case where only one hydraulic cylinder is manufactured.